(1) Field of the Invention
This invention is a solid-state position transducer that can measure the 2-D (x-y) position of a rod moving in a measurement plane perpendicular to the z-axis direction of the rod. The rod has no measureable motion in the z-axis direction. The rod may be the ferromagnetic string of a musical instrument.
(2) State of the Prior Art
U.S. Pat. No. 6,271,456, by the same inventor as the present invention, and which is herein incorporated by reference in its entirety, is directed to a transducer and a musical instrument employing such a transducer. As recognized in that patent, the prior art uses pickups such as elongated electric coil type pickups, which have various problems in reproducing sound that is a true representation of the acoustic properties of the instrument. Other types of known pickups include piezoelectric, strain gauge and accelerometer type electromechanical vibration sensors, but these are also not completely effective in faithfully converting the vibrations of the instrument strings into electrical signals that capture the true sound of the musical instrument. Other solutions recognized in the patent also have various problems. The patent thus proposes using a plurality of magneto resistive elements connected in Wheatstone bridge configurations.
The patent employs a magnetic field that interacts with the with the magneto resistive elements. The magnetic field may be created by a permanent magnet mounted behind the pickup or be generated by a current carried by the string itself. The pickup is positioned so that the vibration of the string causes perturbations in the magnetic field, which in turn alters the resistance of the magneto resistive elements. The electrical pickup of the patent thus senses the position of the vibrating string by measuring changes in the magnetic field applied to opposite sides of a giant magneto resistance (GMR) sensor.
The patent also states that the source of the magnetic field is immaterial, and notes that a permanent biasing magnet could be replaced with a magnetic field carried by the string itself, so that the entire magnetic field would move relative to a sensor. The patent further notes that one way to create this magnetic field is to magnetize the string itself by moving a relative large permanent toward the electrically conductive string, touching the string with the magnet and then slowly moving the magnet away from the string. No technical details of this effect were understood at that time.